Method for monitoring focus in EUV lithography

ABSTRACT

Obtaining optimal focus for exposing a photoresist in an EUV lithography with an EUV mask containing a pattern with an assist feature is disclosed. The EUV mask contains a repeating pattern, wherein the repeating pattern has two different pitches, i.e. a first pitch and a second pitch, and contains an assist feature between main features. Because the two different pitches have different focus offsets, the difference between linewidths of said gratings provides a calibration curve which is a measure of focus. The method for monitoring focus is performing a EUV exposure using a focus position with a pre-determined focus position as calibrated using the linewidth difference between the two gratings. The EUV mask for monitoring focus of present invention is applicable to both test and product masks.

DOMESTIC PRIORITY

This application is a Divisional of the legally related U.S. applicationSer. No. 14/620,803 filed Feb. 12, 2015, which is fully incorporatedherein by reference.

BACKGROUND

This invention relates generally to a method and an EUV mask formonitoring focus in EUV lithography, and more particularly to a methodof obtaining optimal focus for exposing a photoresist covered substratein an EUV lithography system with an EUV mask containing a repeatingpattern with assist features.

In the fabrication of integrated circuits, lithography is used togenerate pattern structures on the semiconductor and various materialsfor the construction of the desired circuit structures. A continuingdemand in view of the ever increasing desire in the semiconductorindustry for higher circuit density in microelectronic devices hasprompted lithographic engineers to develop improved lithographicprocesses. Especially, a lithographic process can provide improvedlinewidth control. Many linewidth variations are due to focusuncertainty caused by numerous effects, such as resist thicknessvariations, bake non-uniformities, batch-to-batch resist sensitivitychanges, non-flat wafers, lens field non-flatness etc. Therefore, toimprove linewidth control one must either improve the focus window ofthe process or reduce the focus variations.

A variety of automated schemes for determining tool focus have beenadopted. In state of the art exposure systems, there is an auto focusleveling sensor system to adjust the wafer in Z direction (perpendicularto resist surface) to achieve the best focus for resist exposures.Although the auto focus leveling sensor system will accurately place thewafers being exposed at the best focus location (or a predeterminedoffset from this position) by high-precision mechanical means, it isstill susceptible to slight drifts in the position mechanism or changesin the required offset for various wafer stacks or various material sets(different photoresists, interlayers and/or underlayers). In addition,variations in imaging and process parameters may cause variations in thebest focus position in the resist film, which in turn may causevariations in the dimensions of printed patterns. Therefore, the focusposition adopted in the exposure system must be continually monitored toensure that the linewidth of printed patterns is tightly controlledwithin an acceptable range.

The most direct method of achieving higher area density is to improvethe resolution of circuit patterns in resist films. One way of improvingthe resolution in resist is to migrate to shorter wavelength from 365 nmto 248 nm, then to 193 and 157 nm, to go to extremely small wavelengthsoptical systems such as EUV (extreme ultraviolet), or to adopt nonoptical system such as E-beam. EUV lithography with exposure wavelengthsbelow 20 nm allows the industry to print features beyond the diffractionlimit of the current 193 nm lithography without resorting to theadoption of tricks using double or triple patterning. Focus variationscan also be caused by diffraction effects due to the three-dimensionalnature of the mask absorber features which are large compared to thewavelength of EUV. The three-dimensional features interact with theincident and reflected electromagnetic field and can cause focusvariations on the wafer. These focus variations mean that it becomeseven more important to have a metric for focus.

Some focus monitoring methods have been reported previously to determinefocus variations during lithographic process. For example, Brunner etal., in U.S. Pat. No. 5,300,786, teach a focus monitoring method thatcan be used to accurately determine the best focus and focal plane in anoptical projection system, but it requires the use of a specialized maskthat cannot be integrated with product masks. The mask requires a 90degree phase shift which is not a standard process. To fabricate 90degree phase shift for EUV mask is very hard, likely involvingoffsetting the multilayer in Z direction, thus it is impractical to addthis feature to a production mask. Although Sun et al. in a 2013 SPIEpaper (Proc. of SPIE, Vol. 8679, pp. 86790T1-12, 2013) demonstrate anEUV mask having phase shift mask structure with 90.9° and 180° phaseshifts for focus monitoring, they also disclose how difficult it is tobuild such phase-shifted targets. This is more evidence that EUV phaseshift mask is too expensive and impractical for actual production of EUVmasks.

In addition, Brunner et al., in U.S. Pat. No. 7,455,939, teach a methodof making a process monitor grating pattern for use in a lithographicimaging system. The method measures focus in a single exposure, and usessidewall angle of a resist measured by scatterometer to determine focus.In EUV lithography, extremely thin resists (˜30 nm) are used with roughsidewall, so the sidewall angle measurement is very hard if notimpossible.

SUMMARY

According to an aspect of the present invention, there is provided amethod for monitoring focus in an EUV lithography system containing:providing a substrate containing a photoresist layer; exposing, with EUVradiation, and printing the photoresist layer at different focuspositions with a mask containing a repeating pattern of two differentpitches which include a first pitch and a second pitch, wherein therepeating pattern of at least one of the two different pitches containsan assist feature; determining a first relationship between printedlinewidths of the photoresist layer and the different focus positionsunder the repeating pattern of the first pitch in the mask; determininga second relationship between printed linewidths of the photoresistlayer and the different focus positions under the repeating pattern ofthe second pitch in the mask; establishing a calibration curve whichcontains printed linewidth differences of the two different pitches atthe different focus positions; and performing EUV exposure onphotoresists or other photo sensitive materials using a focus positionwhich is pre-determined as a focus position on the calibration curve.

Exposing the photoresist layer under the repeating pattern in the maskresults in a printed photoresist pattern selected from a groupconsisting of lines, trenches, holes, studs, and mixtures thereof. Thelines or trenches include horizontal lines or trenches and verticallines or trenches. The assist feature generally has a linewidth in arange from about 20 nm to about 60 nm on the EUV mask, which correspondsto a range from about 5 nm to about 15 nm in wafer dimensions on theprinted photoresist pattern. The assist feature is generally not printedon the photoresist pattern. The photoresist layer used in EUVlithography generally has a thickness in a range from about 10 nm toabout 40 nm. The first pitch and the second pitch are selected from aset of pitches inspected to give a largest or sufficient difference attheir best focus positions for the printed photoresist pattern. Optimumpitches may be different depending on illuminator and can be predictedusing simulation software or experimental results. However, mostilluminators at small pitch will have similar behavior. The illuminationmay be a quasar illumination, and more specifically the illumination maybe a 25° or 45° quasar illumination system with σ_(out)/σ_(in)=088/0.37.Other illumination conditions may be used. The current state-of-the-artEUV exposure system has a numerical aperture (NA) of 0.33. Higher NA isexpected to provide better resolution for printing resist images. Theprojection system of the EUV lithography has a demagnification of 4×, sothe feature sizes of the repeating pattern in the mask are four timesthe feature sizes of the printed photoresist pattern on the substrate.The EUV mask used here for monitoring the focus may be a product mask ora test mask. The product mask is for producing products. The method usedhere is not dependent on photoresist. The photoresist used for EUVlithography may be a chemically amplified resist.

According to another aspect of the present invention, there is providedan EUV mask for monitoring focus in an EUV lithography containing: amask substrate; a multilayer reflector on the mask substrate; and anabsorber over the multilayer reflector, the absorber containing arepeating pattern of two different pitches which include a first pitchand a second pitch, wherein the first pitch and the second pitch areselected from a set of pitches inspected to give a largest or sufficientdifference at their best focus positions for a printed photoresistpattern, and the repeating pattern of at least one of the two differentpitches contains an assist feature.

For the above EUV mask for monitoring focus in EUV lithography, the bestfocus positions for the printed photoresist pattern for the above set ofpitches are determined by: exposing, with EUV radiation, and printing aphotoresist layer at different focus positions with a mask containingthe repeating pattern of the set of pitches; determining relationshipsbetween printed linewidths of the photoresist layer and the differentfocus positions under the repeating pattern of the set of pitches; andidentifying their best focus positions based on the relationships. Theprinted photoresist pattern obtained through an EUV exposure over therepeating pattern of a mask on a photoresist layer, may contain patternsof lines, trenches, holes, studs, and mixtures thereof. The above EUVmask for monitoring focus in EUV lithography may contain a cappinglayer. The multilayer reflector may contain 40 layers of molybdenum (Mo)and silicon (Si) double layer, and the absorber may contain tantalum orother EUV absorbing materials. The absorber may contain Ta, TaN, TaBN,TaBON, TaGeN, Cr, CrO_(x), Ge, Al, Al—Cu, Cu, Al₂O₃, Ti, TiN, SnTe,ZnTe, or mixtures thereof. The assist feature is generally has alinewidth in a range from about 20 nm to about 60 nm in the EUV mask,which corresponds to a range from about 5 nm to about 15 nm on theprinted photoresist pattern.

According to yet another aspect of the present invention, there isprovided a method of making a test pattern in an EUV mask for monitoringfocus in EUV lithography containing: providing a substrate containing aphotoresist layer; exposing, with EUV radiation, and printing thephotoresist layer at different focus positions with a mask containing arepeating pattern of a set of pitches, wherein the repeating pattern ofat least one pitch contains an assist feature; determining relationshipsbetween printed linewidths of the photoresist layer and the differentfocus positions under the repeating pattern of the set of pitches;identifying best focus positions based on the relationships; andselectively etching an absorber of an EUV mask to form the repeatingpattern of two different pitches which include a first pitch and asecond pitch, wherein the first pitch and the second pitch are selectedfrom the set of pitches inspected to give a largest or sufficientdifference at their best focus positions for a printed photoresistpattern.

The above EUV mask with the absorber selectively etched also contains asubstrate, a multilayer reflector, and may also contain a capping layer.The multilayer reflector may contain 40 layers of molybdenum and silicondouble layer, and the absorber may contain tantalum or another EUVabsorbing material. The absorber may contain Ta, TaN, TaBN, TaBON,TaGeN, Cr, CrO_(x), Ge, Al, Al—Cu, Cu, Al₂O₃, Ti, TiN, SnTe, ZnTe, ormixtures thereof. The assist feature generally has a linewidth in arange from about 20 nm to about 60 nm on the EUV mask, which correspondsto a range from about 5 nm to about 15 nm on the scale of wafercoordinates in the printed photoresist pattern. The printed photoresistpattern using the above EUV mask may contain patterns of lines,trenches, holes, studs and mixtures thereof. The EUV mask made here formonitoring the focus may be a product mask or a test mask. The productmask is for producing products. The step of selectively etching anabsorber of an EUV mask may be replaced with a step of: selectivelydepositing an absorber of an EUV mask to form the repeating pattern oftwo different pitches which include a first pitch and a second pitch,wherein the first pitch and the second pitch are selected from the setof pitches inspected to give a largest or sufficient difference at theirbest focus positions for a printed photoresist pattern.

According to still yet another aspect of the present invention, there isprovided a method of monitoring focus in an EUV lithography systemcontaining: providing a substrate containing a photoresist layer;exposing, with EUV radiation, and printing the photoresist layer atdifferent focus positions with a mask containing a repeating pattern oftwo different pitches which include a first pitch and a second pitch,wherein the repeating pattern of at least one of the two differentpitches contains an assist feature; determining a first relationshipbetween printed linewidths of the photoresist layer and the differentfocus positions under the repeating pattern of the first pitch in themask; determining a second relationship between printed linewidths ofthe photoresist layer and the different focus positions under therepeating pattern of the second pitch in the mask; identifying areference focus position where a printed linewidth of the first pitch issame as a printed linewidth of the second pitch; and performing EUVexposure on photoresists or other photo sensitive materials using afocus position with a pre-calibrated offset from the identifiedreference focus position.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional diagram representing an assistfeature on an ArF Mask and on an EUV mask according to one embodiment ofthe present invention.

FIG. 2 is a flow chart of a method of making a test pattern in an EUVmask for monitoring focus in EUV lithography according to one embodimentof the present invention.

FIG. 3 is a schematic cross-sectional diagram representing a repeatingpattern of two different pitches with a single assist feature betweenmain features on a mask for EUV according to one embodiment of thepresent invention.

FIG. 4 is a flow chart of a method for monitoring focus in an EUVlithography apparatus according to one embodiment of the presentinvention.

FIG. 5 is a plot of experimental linewidth vs. focus for differentexposure doses for vertical (left) and horizontal (right) trenches at apitch of 72 nm on the wafer with no assist features according to oneembodiment of the present invention.

FIG. 6 is a plot of experimental linewidth vs focus for differentexposure doses for vertical (left) and horizontal (right) trenches at apitch of 72 nm on the wafer with assist features of nominal width of 10nm in wafer dimensions between the main trenches according to oneembodiment of the present invention.

FIG. 7 is a plot of simulations of linewidth vs focus for the 72 nm andthe 112 nm pitch trenches with 10 nm wide assist features between themain features, and a calibration curve plot which contains thedifference in linewidth between the 72 nm and the 112 nm pitch featurescan be used as a signal indicating focus position according to oneembodiment of the present invention.

DETAILED DESCRIPTION

This invention relates generally to a method and an EUV mask formonitoring focus in EUV lithography. The invention also relates to amethod of making the EUV mask for monitoring focus in EUV lithography.Specifically, the focus monitoring method provides a way to obtain anoptimal focus for exposing a photoresist in an EUV lithography systemwith an EUV mask containing a pattern with an assist feature.

To differentiate from regular X-ray, the EUV wavelength is usuallyreferred to as being in a range from about 5 nm to about 40 nm. Thisrange overlaps with the soft X-ray range which is generally in a rangefrom about 2 nm to about 20 nm. The current EUV lithography uses an EUVsource at around 11 nm to 14 nm, more specifically at 13.5 nm. Since thewavelength is much shorter than the current 193 nm optical system usedin the industry, the potential advantage of being capable of resolvingmuch smaller dimension of photoresist images should be realized with theadoption of this technology.

Referring to FIG. 1, the 193 nm (ArF wavelength) photolithography usesan imaging system which directs an ArF radiation 13 through a mask, andthen through a 4× (or 5×) demagnification projection system to projectthe image of that pattern onto a photoresist layer on top of asemiconductor wafer for later development and etch. For EUV lithographicsystem, the EUV radiation 14 is reflected from a mask to form a pattern,then the reflected light 15 and 16 containing the image of that patternis projected through a 4× demagnification projection system onto aphotoresist layer on top of a semiconductor wafer. In addition, theremay be an ‘anamorphic’ system with demagnification of 4× in onedirection and 8× in the other direction. The regular ArF mask contains amask substrate 10 with an absorber 20 over the substrate, while the EUVmask contains a mask substrate 40, a multilayer reflector 50 and anabsorber 60. The material for the ArF mask substrate 10 is usually highquality fused quartz, and the material for the absorber 20 may bechromium or OMOG (opaque MoSi on glass). The material for the EUV masksubstrate 40 is usually low thermal expansion quartz which may be dopedwith titania (TiO₂). The multilayer reflector 50 may contain 40 layersof molybdenum and silicon double layer. On top of the multilayerreflector the stack may contain a capping layer (not shown in FIG. 1, itwould be between layers 50 and 60) which is usually ruthenium. Theabsorber 60 is generally a tantalum based material and is optimized for193 nm inspection and to reduce EMF (electromagnetic field) effect. Itmay be Ta, TaN or TaBN material. An exemplary assist feature (˜40 nmhere) size in the ArF lithography as shown in the mask opening 30 ismuch smaller than the ArF wavelength 11, while the same assist featuresize in the EUV lithography as shown in the mask opening 70 is muchlarger than the EUV wavelength 12. This big difference between ArFlithography and EUV lithography results in different diffraction effectin the photolithographic process, especially when the masks have assistfeatures. While the absorber thickness and width are smaller than thewavelength of light in the case of ArF and the mask can be considered‘thin’ compared to the wavelength, in the case of EUV both the lateraldimension and the absorber thickness are larger than the wavelength,thus creating a ‘thick’ mask. In addition, for the EUV mask the lighttravels along the absorber twice because EUV uses reflective masks.While the light travels along the absorber edge it interacts with theabsorber which has a refractive index smaller than that of vacuum, thusaccelerating the light along the absorber edge. For the smaller assistfeature this effect is stronger than for the larger main feature, thuscausing the different focus properties for main features with andwithout assist features. Assist features are usually sub-resolutionfeatures being used to enhance the image quality or process window ofthe main features. They are placed in proximity of the main features onthe mask and are generally not printed in the final photoresist images.They usually have the same or similar shape as the main features, suchas lines, trenches, bars, holes . . . etc but are smaller in size. Theassist feature generally has a linewidth (or spacewidth) in a range fromabout 20 nm to about 60 nm in the EUV mask, which corresponds to a rangefrom about 5 nm to about 15 nm on the printed photoresist pattern. Theassist feature is generally not printed on the photoresist pattern. Theterm “linewidth” represents the width of any feature not just of a linefeature. For a line feature, it is the width of the line. For a trenchfeature, it is the width of the trench or space, and may be called aspacewidth or trenchwidth. For a contact hole or stud feature, it is thediameter of the hole or stud. The feature includes main feature andassist feature. Since the drawing in FIG. 1 is intended for illustrativepurpose, the drawing is not necessarily drawn to scale. The drawing alsoonly shows a few layers of the multilayer reflector, and does not showthe entire 40 layers of the Mo—Si. The absorbers may contain ARC(antireflective layer), and the substrate may contain other layer orlayers, such as CrON layer. These are not shown in the drawing in FIG. 1either.

In addition to the absorber opening 70 being generally larger than theEUV wavelength 12 as shown in FIG. 1, the thickness of the absorber isalso generally larger than both the assist opening 70 and the EUVwavelength 12. The opening 70 is in vacuum, thus the real part of therefractive index n_(vac) is 1 and the imaginary part of the refractiveindex k_(vac) is 0. The typical absorber 60 currently used in theindustry has the real part of the refractive index n_(abs) around 0.90to 0.96, and has the imaginary part of the refractive index k_(abs)around approximately 0.03 or higher. The difference between n_(vac) andn_(abs) here is sufficient to cause a significant phase shift whenassist features are introduced to the EUV mask, thus causing a focusdrift between different pitches in the printed photoresist images. Thepresent invention provides a method to monitor the focus in EUVlithography to address this focus shift issue, and at the same time theinvention uses this focus shift property in the monitoring method. Thefocus monitoring method is to create a mask with two test gratings, andthen it uses the focus shift of the printed resist images under thesetwo gratings to adjust the focus. The preferred test gratings arehorizontal or vertical trenches. At least one of the test gratingsrequires an assist feature, but the present invention can be used tomonitor focus for a desired printing pattern with or without assistfeatures.

One embodiment of the present invention includes a method of making atest pattern in an EUV mask for monitoring focus in EUV lithography. Themethod is described in the flow chart of FIG. 2. At block 201 of FIG. 2,a substrate containing a photoresist layer is provided. The substrate isusually a semiconductor substrate, for example a silicon wafer. Thesubstrate may contain other materials under photoresist layer, forexample organic bottom antireflective coating (BARC), siliconantireflective coating (SiARC), underlayer (UL), or combinationsthereof. At block 202 of FIG. 2, the photoresist layer is exposed withEUV radiation at different focus positions with a mask containing arepeating pattern of a set of pitches, wherein the repeating pattern ofat least one pitch contains an assist feature. In many instances, theexposure also contains various doses. Combining different focuses andexposure doses, usually called Focus-Exposure Matrix (FEM), to determinethe best process condition is common in the lithographic community. Thethickness of the photoresist layer may be in a range from about 10 nm toabout 40 nm, preferably from about 20 nm to about 35 nm. The photoresistlayer after exposure is then processed with a commonly known photoresistprocess, such as baking and then developing with a developer, to form aprinted photoresist image on the substrate. The photoresist may be achemically amplified photoresist or a non-chemically amplifiedphotoresist. The photoresist for EUV lithography may also be eitherpositive tone or negative tone. The repeating pattern may be any typesuch as lines, trenches, holes, and studs. The shape may also vary forthe same type, for example a hole may be round, oval, square orrectangular. The repeating pattern may also contain mixtures ofdifferent types or shapes. In the present invention, finding the optimumpitches can be done either experimentally or by simulation. For thesimulation, a commercial software tool is used that is able to calculatethe exact diffraction orders that are reflected off the mask. The tooluses a description of the incident light, in one example case a quasarillumination, and the information of the exact layer stack andcalculates the electromagnetic field that surrounds the mask, includingthe multi layer stack and the absorber materials. The reflecteddiffraction orders are propagated in the same simulation tool to predictthe incident intensity in the wafer plane, and are then able to predictthe linewidth of the resulting resist image. Such simulation tools areubiquitous in the lithography industry today and they are very good atpredicting trends and sensitivities, such as relative focus shifts,while being less good at predicting absolute numbers. Such simulationscan be used to select the best pitches for maximum focus shift, and canthus replace the experimental methods.

The preferred repeating pattern for the present invention is a repeatingpattern capable of printing a group of repeating lines or trenches, i.e.gratings. The lines or trenches may be vertical, horizontal or both. Therepeating pattern on the EUV mask for printing the test gratings maycontain an assist feature. As shown in FIG. 3, a schematiccross-sectional diagram of an EUV mask for printing trenches withpositive photoresist, there are two assist feature openings 70 on eachside of a main trench feature opening 80. In the printing process theassist feature will not be printed on the photoresist layer. The printedphotoresist images will contain a group of repeating trenches with apitch which is one fourth of the mask pitch 17. FIG. 3 only shows 3trench sets of the repeating pattern of pitch 17 for illustrativepurpose on the left, and 3 trench sets of the repeating pattern of pitch18 on the right. The grating with pitch 18 on the right may haveslightly different main feature openings 85 and slightly differentassist feature openings 75, or it may have no assist feature openings atall. The EUV mask used in the embodiment here contains a set of pitcheswith many trenches in each pitch. Choice of the pitch sizes of the setof pitches may be based on the pitch range of the product wafer (or testwafer) to be monitored. The number of pitches of the set of pitchesshould be at least 3. In some instances, 2 pitches are predetermined,and then there is no selection process. The repeating pattern maycontain no assist features in some pitches of the set of pitches.Besides mask substrate 40, multilayer reflector 50 and absorber 60, FIG.3 also shows a capping layer 90. Regular EUV mask preferably includes acapping layer. Since the drawing in FIG. 3 is intended for illustrativepurpose, the drawing is not necessarily drawn to scale. The drawing alsoonly shows a few layers of the multilayer reflector, and does not showthe entire 40 layers of the Mo—Si double layer.

At block 203 of FIG. 2, the relationships between printed linewidths (orspacewidths for trenches) of the photoresist layer and the differentfocus positions under the repeating pattern of the set of pitches aredetermined. To determine the relationship between printed linewidth andfocus, a plot of linewidth versus focus is constructed. The linewidthsare measured from the printed photoresist images with scanning electronmicroscope (SEM), atomic force microscopy (AFM), or scatterometry. Eachrelationship is determined by each plot of linewidth vs. focus at eachpitch of the set of pitches chosen in the investigation.

At block 204 of FIG. 2, the best focus positions based on therelationships are identified. The plot of linewidth vs. focus is usuallycalled Bossung plot. In 1977, J. W. Bossung first described a plottingmethod to determine the best focus and exposure dose (which he calledexposure energy) for maintaining the Critical Dimensions (CDs) ofprinted patterns from lithography. Ever since then, Bossung Plots havebeen used to analyze the CD variation of a printed feature vs. the focuslatitude and the exposure latitude. The Bossung Plot usually exhibitseither “smile” (turn upward across focus positions) or “frown” (turndownward across focus positions), except at the isofocal dose, the curveis almost flat and relatively insensitive to changes in focus positions.The bottom of the smile or the top of the frown is the best focusposition. The best focus positions are identified based on the BossungPlots for the different pitches in the set of pitches.

At block 205 of FIG. 2, an absorber of an EUV mask is selectively etchedto form the repeating pattern of two different pitches, wherein a firstpitch and a second pitch are selected from the set of pitches inspectedto give a largest or sufficient difference at their best focus positionsfor a printed photoresist pattern. In the selected two pitches whichinclude the first pitch and the second pitch, the repeating pattern ofat least one pitch contains an assist feature. As shown in FIGS. 1 and3, the EUV mask may contain a mask substrate 40, a multilayer reflector50 and an absorber 60. The material for the EUV mask substrate 40 may beany substrate which has low thermal expansion, such as ultralowexpansion titanium silicate glass. The multilayer reflector 50 maycontain 40 layers of molybdenum and silicon alternating double layer.Other materials may alternatively be used for the multilayer reflector50, such as a Mo/Be multilayer stack, a Ru/Si multilayer stack, aSi/Mo/Ru multilayer stack, a Si/Mo/Sr multilayer stack, a Si/Mo/Ru/Momultilayer stack, and a Si/Ru/Mo/Ru multilayer stack. The multilayerreflector 50 may contain from about 20 to about 80 layers of molybdenumand silicon alternating double layer. The thickness of the alternatinglayer may be in the range from about 2 nm to about 10 nm. One specificexample, the alternating layers of Mo and Si with thickness of ˜4.1 nmand ˜2.8 nm respectively are deposited onto mask substrate 40 to buildup to 40 or more pairs each having thickness of ˜6.9 nm (4.1+2.8) for13.40 nm EUV wavelength operation. The total thickness of the multilayerreflector 50 may be in the range from about 160 nm to about 800 nm. Theabsorber 60 may contain Ta, TaN, TaBN, TaBON, TaGeN, Cr, CrO_(x), Ge,Al, Al—Cu, Cu, Al₂O₃, Ti, TiN, SnTe, ZnTe, or mixtures thereof. On topof the multilayer reflector the stack may contain a capping layer 90which is usually ruthenium. Other materials may be used for the cappinglayer 90, such as SiO₂, C, Cr, Rh, or CrN. In some instances, the EUVmask may further contain a conductive coating (not shown in FIG. 1 or 3)formed on a back side of the substrate 40 to allow for electrostaticchucking. The conductive coating may contain Cr or CrN, or othersuitable materials. An EUV mask with the absorber 60 not patterned isetched using any suitable etching technique. In general, a photoresistlayer is patterned on top of the absorber and that pattern is replicatedinto the absorber by etching the absorber in locations not covered bythe photoresist layer to form the mask features, such as the patternedfeatures shown in FIG. 3. The etching process is generally a processwith reactive etching using proper etchant. Alternatively, the absorber60 with desired mask features is deposited on an EUV mask which has noabsorber 60 on top of the multilayer reflector 50. Generally, aphotoresist layer is patterned on top of the multilayer reflector 50 andthat pattern is replicated onto the multilayer reflector 50 bydepositing the absorber in locations not covered by the photoresistlayer to form the mask features, such as the patterned features shown inFIG. 3. Various deposition processes include but are not limited to:physical vapor deposition, chemical vapor deposition and electrochemicaldeposition. After etching or depositing, the photoresist layer is thenstripped with the method commonly known in the industry. The patternetched or deposited here is the repeating pattern of two differentpitches, wherein a first pitch and a second pitch are selected from theset of pitches inspected to give a largest or sufficient difference attheir best focus positions for a printed photoresist pattern. The bestfocus positions for a printed photoresist pattern for the set of pitchesare determined at the step of block 204 of FIG. 2. The first pitch andthe second pitch are selected within this set of pitches to give alargest or sufficient difference at their best focus positions.Sometimes the largest difference is so big that the second largest orthird largest may be sufficient. Large difference at their best focuspositions generally results in large difference in printed linewidths.In this case, any two pitches with sufficient difference at their bestfocus positions may be selected. The sufficient difference in thisinvention represents a difference of about 35 nm or larger. Optimumpitches may be different depending on illuminator. However, mostilluminators at small pitch will have similar behavior. The illuminationmay be a 25° quasar illumination system with □_(out)/□_(in)=088/0.37.Quasar is a quadrupole illumination system. Generally, the regions ofthe quadrupole illumination are not square in shape, but are oftencircular or annular sections. The illumination may also be a 45° quasaror other angles. Other illumination condition may also be used for otherlithographic requirement. The current EUV stepper has an NA of 0.33.Higher NA is contemplated for future generation EUV tool set. Theprojection system of the EUV lithography has a demagnification factor of4×, so the feature sizes of the repeating pattern in the mask are fourtimes the feature sizes of the printed photoresist pattern on thesubstrate. Larger demagnification factors, such as 8×, and evenanamorphic systems with one magnification in one direction and a secondmagnification in the other direction are contemplated in the industry.The repeating pattern of at least one of these two pitches in EUV maskcontains an assist feature. The assist feature generally has a linewidth(or spacewidth) in a range from about 20 nm to about 60 nm in the EUVmask, which corresponds to a range from about 5 nm to about 15 nm on theprinted photoresist pattern. Exemplary assist feature 70 is shown inFIG. 3. The EUV mask containing the repeating pattern of these twopitches described above in the absorber 60 acting as a special focustest target is useful for monitoring focus in EUV lithography. The EUVexposure may then be performed using a focus position pre-calibratedbased on the relationships between printed linewidth and focus of thesetwo pitches. The EUV exposure may be performed on a photoresist over asubstrate. The substrate is generally a semiconductor substrate, forexample a silicon wafer. The EUV exposure may also be performed on otherphoto sensitive materials. The sequence of the steps in the flow chartin FIG. 2 is preferred. However, the invention is not limited to theperformance of these steps with the sequence or order presented in thisflow chart. Many steps may also be applied to the substrate before,between or after the steps shown in this flow chart.

Another embodiment of the present invention includes an EUV mask formonitoring focus in EUV lithography. An exemplary EUV mask formonitoring focus in EUV lithography may contain a mask substrate whichgenerally has low thermal expansion, such as ultralow expansion titaniumsilicate glass. On top of the substrate the stack may contain amultilayer reflector which may contain 40 layers of molybdenum andsilicon alternating double layer. On top of the multilayer reflector maycontain a capping layer which may be ruthenium. On top of the cappinglayer it may contain an absorber which may contain Ta, TaN, TaBN, TaBON,TaGeN, Cr, CrO_(x), Ge, Al, Al—Cu, Cu, Al₂O₃, Ti, TiN, SnTe, ZnTe, ormixtures thereof. The absorber contains a repeating pattern of twodifferent pitches, wherein a first pitch and a second pitch are selectedfrom a set of pitches inspected to give a largest or sufficientdifference at their best focus positions for a printed photoresistpattern, and the repeating pattern of at least one of the two differentpitches comprising an assist feature. Optimum pitches may be differentdepending on illuminator. However, most illuminators at small pitch willhave similar behavior. The illumination may be a 25° quasar illuminationsystem with σ_(out)/σ_(in)=088/0.37. Other illumination condition mayalso be used for other lithographic requirements. The current EUVstepper has an NA of 0.33. Higher NA is contemplated for futuregeneration EUV tool set. The printed photoresist pattern obtainedthrough the EUV exposure over the repeating pattern of the EUV mask on aphotoresist layer may be lines, trenches, holes, studs or mixturesthereof. The printed photoresist pattern is generally trench gratings.The representative repeating pattern of the EUV mask may be referred toFIG. 3. To determine their best focus positions of the printedphotoresist pattern is by the following steps: (1) exposing with EUVradiation and printing the photoresist layer at different focuspositions with a mask comprising the repeating pattern of the set ofpitches; (2) determining relationships between printed linewidths of thephotoresist layer and the different focus positions under the repeatingpattern of the set of pitches; and (3) identifying the best focuspositions based on the relationships. Choice of the pitch sizes of theset of pitches may be based on the pitch range of the product wafer tobe monitored. The number of pitches of the set of pitches should be atleast 3. Each relationship is determined by each plot of linewidth (orspacewidth for trenches) vs. focus at each pitch of the set of pitcheschosen in the investigation. The best focus positions are identifiedbased on the Bossung Plots for the different pitches in the set ofpitches. The repeating pattern of at least one of these two pitches inEUV mask contains an assist feature. The assist feature generally has alinewidth in a range from about 20 nm to about 60 nm in the EUV mask,which corresponds to a range from about 5 nm to about 15 nm on theprinted photoresist pattern. The EUV mask for monitoring focus in EUVlithography may further contain a conductive coating formed on a backside of the substrate to allow for electrostatic chucking. Theconductive coating may contain Cr or CrN. The EUV mask for monitoringfocus in EUV lithography may be either a test mask or a product mask. Aproduct mask is used for exposing product wafers and producing products,and sometimes is also called production mask. A test mask is only fortesting, not for producing products. By placing the repeating pattern ofthe first pitch and the second pitch at specific locations outside theactive area of a product mask, the EUV mask for monitoring focus in EUVlithography of the present invention can be a product mask. Therepeating pattern of these two pitches described above acting as aspecial focus test target is then used for monitoring focus of thepattern in the active area in EUV lithography. The EUV exposure may beperformed using a focus position pre-determined based on therelationships between printed linewidth and focus under these twopitches.

Yet another embodiment of the present invention includes a method ofmonitoring focus in an EUV lithography system. The method is describedin the flow chart of FIG. 4. At block 401 of FIG. 4, a substratecontaining a photoresist layer is provided. The substrate is usually asemiconductor substrate, for example a silicon wafer. The substrate isalso suitably any substrate conventionally used in the semiconductorprocess. For example, the substrate can be silicon, silicon oxide,aluminum-aluminum oxide, gallium arsenide, ceramic, quartz, copper orany combination thereof, including multilayers. The substrate caninclude one or more semiconductor layers or structures and can includeactive or operable portions of semiconductor devices. The substrate maycontain other materials under photoresist layer, for example organicBARC, SiARC, UL, or combinations thereof. The photoresist layer isusually formed on the substrate through a process including a spincoating step and a baking step. The photoresist may be a chemicallyamplified photoresist or a non-chemically amplified photoresist. Thephotoresist for EUV lithography may also be either positive tone ornegative tone. The thickness of the photoresist layer may be in a rangefrom about 10 nm to about 40 nm, preferably from about 20 nm to about 35nm.

At block 402 of FIG. 4, the photoresist layer is exposed with EUVradiation and printed at different focus positions with a maskcontaining a repeating pattern of two different pitches, wherein therepeating pattern of at least one pitch contains an assist feature. Themask is an EUV mask for monitoring focus in EUV lithography as describedin other embodiments of the present invention. The absorber of the EUVmask for monitoring focus in EUV lithography contains a repeatingpattern of two different pitches, wherein a first pitch and a secondpitch are selected from a set of pitches inspected to give a largest orsufficient difference at their best focus positions for a printedphotoresist pattern, and the repeating pattern of at least one pitchcontaining an assist feature. The photoresist layer after exposure isthen processed with a typical photoresist process, such as baking andthen developing with a developer, to form printed photoresist images onthe substrate. The repeating pattern may be any type such as lines,trenches, holes, and studs. The shape may also vary for the same type,for example a hole may be round, oval, square or rectangular. Therepeating pattern may also contain mixtures of different types orshapes. The preferred repeating pattern for the present invention is arepeating pattern capable of printing a group of repeating lines ortrenches, i.e. gratings. The lines or trenches may be vertical,horizontal or both. For example, vertical trenches are a group ofrepeating trenches aligned parallel in the vertical direction, while thehorizontal trenches are aligned perpendicular to the vertical direction.The repeating pattern of at least one pitch on the EUV mask formonitoring focus in EUV lithography for printing the test gratingscontains an assist feature. An example of the repeating pattern as shownin FIG. 3, a schematic cross-sectional diagram of an EUV mask forprinting trenches, there are two assist feature openings 70 on each sideof a main trench feature opening 80. In the printing process, the assistfeature will not be printed on the photoresist layer. The printedphotoresist images will contain a group of repeating trenches with apitch which is one fourth of the mask pitch 17 for the first selectedgrating, and another set of repeating trenches with a pitch which is onefourth of the mask pitch 18.

At block 403 of FIG. 4, a first relationship is determined betweenprinted linewidths (or called spacewidths for trenches) of thephotoresist layer and the different focus positions under the repeatingpattern of the first pitch in the mask. The first relationship isdetermined by a plot (Bossung Plot) of linewidth vs. focus at the firstpitch. At block 404 of FIG. 4, a second relationship is determinedbetween printed linewidths of the photoresist layer and the differentfocus positions under the repeating pattern of the second pitch in themask. The second relationship is determined by a plot of linewidth vs.focus at the second pitch.

At block 405 of FIG. 4, a calibration curve which comprises printedlinewidth differences of the two different pitches at different focuspositions is established. The calibration curve is obtained bycalculating the differences in linewidths for the two different pitches,and then linewidth differences are plotted versus focus positions. Thiscalibration curve is then used as a reference curve for calibratingfocus for any other focus positions of interest. The focus position withzero difference in linewidth is the position where the Bossung Plot ofthe first pitch intersects the Bossung Plot of the second pitch. Anyfocus position of interest can be either in the positive direction or inthe negative direction in this calibration curve.

At block 406 of FIG. 4, an EUV exposure is performed using a focusposition which is pre-determined as an optimal focus position on thecalibration curve. An optimal focus position is a position of interest,which may be determined by the user of this invention. The embodiment ofthe present invention here provides a method for monitoring focus in EUVlithography, and more particularly to a method of obtaining optimalfocus for exposing a photoresist in an EUV lithography system. Theoptimal focus position may or may not be at the focus position with zerodifference in linewidth on the calibration curve. The optimal focusposition may be determined through the regular means such as BossungPlots of a desired test pattern or a desired product pattern. Theoptimal focus position may also be determined by the desired imagefidelity or the desired specific linewidth of the image. For someinstances, the optimal focus position may not provide the best imagequality, but provides a special image profile such as tapered profile orundercut profile. The focus position of the pattern to be monitored maybe very different from these test gratings. For example, the patterns tobe monitored may not have the same kind of assist feature, or may nothave assist features at all, thus would have very different focuspositions of these two test gratings. After the optimal focus positionis determined, a focus position on the calibration curve of this optimalfocus position can be located. The EUV exposure may then be performed ona photoresist over a semiconductor substrate using a focus positionwhich is pre-determined as the optimal focus position on the calibrationcurve. The EUV exposure may also be performed on other photo sensitivematerials. Exemplary photo sensitive materials are photosensitivedielectrics and developable bottom antireflective coatings (DBARC). FIG.4 and the associated steps 401 through 406 describes the calibrationsequence so that the focus can be determined by just looking at thewidths of the lines of the two different pitch gratings. Once thiscalibration curve is recorded, one can then determine focus for eachwafer that may be exposed during regular manufacturing process and nomore through-focus or through-dose experiments have to be performed. Thesequence of the steps in the flow chart in FIG. 4 is preferred. However,the invention is not limited to the performance of these steps with thesequence or order presented in this flow chart. Many steps may also beapplied to the substrate before, between or after the steps shown inthis flow chart.

At block 405 and 406, a calibration curve is used as a reference curvefor calibrating focus for any other focus positions of interest.Alternatively, a reference focus position is identified at a focusposition where a printed linewidth of the first pitch is the same as aprinted linewidth of the second pitch. The reference focus position isthe position where the Bossung Plot of the first pitch intersects theBossung Plot of the second pitch, where the linewidth difference betweenthe two pitches is zero. This intersect position is then used as areference focus position for any other focus positions of interest. AnEUV exposure is then performed on a photoresist over a semiconductorsubstrate using a focus position with a pre-calibrated offset from thisidentified reference focus position. The EUV exposure may also beperformed on other photo sensitive materials. The embodiment of thepresent invention here provides a method for monitoring focus in EUVlithography, and more particularly to a method of obtaining optimalfocus for exposing a photoresist in an EUV lithography. The optimalfocus position may or may not be the same as the reference focusposition. The optimal focus position may be determined through theregular means such as Bossung Plots of a desired test pattern or adesired product pattern. The optimal focus position may also bedetermined by many reasons as described in the previous paragraph. Afterthe optimal focus position is determined, a calibrated offset of thisoptimal focus position to the identified focus position can be obtained.This offset value only needs to be calibrated once in the process. TheEUV exposure may then be performed on a photoresist over a semiconductorsubstrate using a focus position with this pre-calibrated offset fromthe identified reference focus position. The EUV exposure may also beperformed on other photo sensitive materials.

Some specific examples are given below to illustrate the embodiments ofthe present invention. Since these examples are given for illustrativepurpose only, the invention is not limited to the specific details ofthe examples.

Example 1

Referring to FIG. 5, the plot shows the experimentally measured width oftrenches through focus, for seven different exposure doses. The leftplot shows values for vertical trenches, the right plot shows values forhorizontal trenches. The dose multiplier relative to the center dose isshown in the legend for each of the curves. A dose multiplier of 1.03,for example, indicates a dose that is 3 percent higher than the centerdose. A higher dose leads to a larger width for the printed trench. Thepitch of the structures that are measured on this wafer is 72 nm, whilethe measured width of the trenches is between about 15 and 25 nm. Thepitch of the openings on the mask is four times the pitch on the wafer,or 288 nm in this case. There are no assist features between the mainfeatures. This is the reason for the strong variations in width throughfocus for the Bossung curves shown here. The mask is illuminated withquadrupole illumination that is described in the previous paragraphs.

Example 2

Referring to FIG. 6, the plot shows experimentally measured width oftrenches through focus for printed trenches of the same pitches andexposure conditions as shown in FIG. 5, with the only difference beingthe addition of assist features between the main features on the mask.The best focus, defined as the focus for which the slope of thelinewidth vs focus curve is horizontal, is moved to much more positivevalues for the assist feature case shown in FIG. 6 compared to the bestfocus in FIG. 5.

Example 3

Referring to FIG. 7, the plot shows the simulated focus response of thetrench width for only the best dose. Shown is the focus response for a72 nm pitch grating with 10 nm assist features, and a 112 nm pitchgrating with 10 nm assist features, as well as the difference betweenthe linewidth at the two pitches, with the scale to the right. When thedifference between the two linewidths is taken, it shows a monotonicallyincreasing signal with focus which can be calibrated for a processaccording to the flow chart in FIG. 4. This monotonic function oflinewidth difference vs. focus then serves as a definite metric so thatthe focus can be determined for any exposed wafer by just measuring twolinewidths for the two different pitches and taking the difference. Forthe simulation results shown in FIG. 7, a commercial software tool isused that is able to calculate the exact diffraction orders that arereflected off the mask. Such simulations can be used to select the bestpitches for maximum focus shift, and can thus replace the experimentalmethods described in steps 201 and 202 of FIG. 2. Because the simulationmethod described here is good at predicting trends rather than absolutenumbers, the flow shown in FIG. 4 for calibration of the focus responseof an existing pair of grating pitches can likely not be replaced withsimulations unless the simulation tool is calibrated exactly to thewafer process that is used in manufacturing.

What is claimed is:
 1. A method of making a test pattern in an EUV maskfor monitoring focus in an EUV lithography comprising: providing asubstrate containing a photoresist layer; exposing, with EUV radiation,and printing the photoresist layer at different focus positions with amask comprising a repeating pattern of a set of pitches, wherein therepeating pattern of at least one pitch comprises an assist feature;determining relationships between printed linewidths of the photoresistlayer and the different focus positions under the repeating pattern ofthe set of pitches; identifying best focus positions based on therelationships; and selectively etching or depositing an absorber of anEUV mask to form the repeating pattern of two different pitches, whereina first pitch and a second pitch are selected from the set of pitchesinspected to give a largest or sufficient difference at their best focuspositions for a printed photoresist pattern.
 2. The method of claim 1,wherein selectively etched or deposited EUV mask comprises a substrate,a multilayer reflector, and the absorber.
 3. The method of claim 2,wherein the selectively etched or deposited EUV mask further comprises acapping layer.
 4. The method of claim 2, wherein the multilayerreflector comprises 40 layers of molybdenum and silicon double layer. 5.The method of claim 2, wherein material of the absorber is selected forma group consisting of Ta, TaN, TaBN, TaBON, TaGeN, Cr, CrO_(x), Ge, Al,Al—Cu, Cu, Al₂O₃, Ti, TiN, SnTe, ZnTe, and mixtures thereof.
 6. Themethod of claim 2, wherein the selectively etched or deposited mask is aproduct mask for producing products.
 7. The method of claim 1, whereinthe assist feature has a linewidth in a range from about 20 nm to about60 nm.